PIT-MEX was operated with 757 equipment....and was rarely ever booked over 30-40. I think that is why they are starting with CLT and will expand to PHL if the loads are worth it.
If my memory serves me correctly, US flew the PHL-MEX & PHL-MEX route for about 18 months in 1995 and 1996, before abandoning the effort because it was unprofitable.